Railway vehicles of parallel cardan drive system transmit torques generated by main electric motors which are fixed to bogies, to rotation axes through flexible couplings and gear devices to rotate vehicle wheels disposed on axles, thereby causing the railway vehicles to travel on rails. Each of the gear devices to be used for this kind of railway vehicles (hereinafter simply referred to as a “gear device”) is equipped with a helical gear wheel (large gear) (hereinafter simply referred to as a “gear wheel”) and a helical pinion (small gear) (hereinafter simply referred to as a “pinion”) which are rotated in gearing mesh with each other to thereby transmit torque. In this case, the pinion fixed to a rotation axis which is coupled to a flexible coupling, and the gear wheel fixed to a drive axle is housed in a gear box. Rotation axis portions located on both axial sides of the pinion are rotatably supported by the gear box through respective first tapered roller bearings, and drive axle portions located on both axial sides of the gear wheel are rotatably supported by the gear box through respective second tapered roller bearings.
In this kind of gear device, in case the viscosity of lubricating oil to be supplied to the tapered roller bearings changes due, for example, to the change in temperature, thereby giving rise to poor lubrication, there will occur such a problem as a seizure of the tapered roller bearings. As a solution, this kind of gear device is assembled such that the axial clearance between an inner ring and an outer ring, respectively, of each of the first and the second tapered roller bearings and the respective rollers, i.e., the alignments of the so-called end play value (σs) are performed by means of shims. As compared with end play values (20 to 30 μm) in ordinary industrial gear devices, it is normal practice to assemble the railway vehicle gear devices with very large end play values (e.g., σs: 60 to 170 μm on the pinion side and σs: 80 to 210 μm on the gear wheel side). In case end play value is set to a large value, rattling about the rotation axis of the tapered roller bearings increases and, accompanied thereby, the inclination of the rotation axis in the axial center of the pinion also increases. As a result, at the time of traveling of the vehicle (at the time of rotation of the pinion), accompanied by the vibrations of the flexible coupling, large precession occurs to the pinion, thereby bringing about bad effect on the gearing mesh of the pair of the gear wheel and the pinion.
It is to be noted here that the end play value is set still larger on the side of the gear wheel as compared with the end play value of the pinion. Ordinarily, the drive axle itself to which the gear wheel is fixed is larger in axial length, and is supported on the rails by means of wheels which are fitted onto an outside of each side of the drive axle. For the above-mentioned reasons and for other reasons, even if the end play value is set to a larger value, there can be considered little or no effect on the gearing mesh of the gear wheel and the pinion that make a pair. Such being the case, with respect to the gear wheel, tooth profile modification on the tooth surface in the tooth depth direction, and crowning and relieving modifications on the tooth surface in the flank line direction are not conventionally performed. Instead, it is normal practice to perform, with respect only to the pinion, tooth profile modification on the tooth surface in the tooth depth direction, and crowning and relieving modifications on the tooth surface in the flank line direction to thereby improve the gearing mesh between the pinion and the gear wheel and the pinion.
In other words, with respect, e.g., to tooth profile modifications are performed over an entire face width in the flank line direction in a uniform shape, e.g., in a predetermined tooth tip range and in a predetermined tooth root range by predetermined values (e.g., 20 to 30 μm in case of 70 mm of face width); crowning is performed on the tooth surface in the flank line direction by a predetermined value (e.g., 20 μm) of an arc curve (e.g., radius R≈17685 mm) over the central area in the face width direction of the tooth surface; and also relieving (modification) of a predetermined value (e.g., 50 μm) is performed on both sides in the face width direction by an arc curve of a radius that is different from the arc curve at the time of the crowning. In this manner the two-dimensional (2-D) tooth surface modification is performed by making, to serve as a predetermined value (e.g., 70 μm), the amount of modification in the flank line direction by adding the crowning and relieving on both end surfaces in the face width direction.
By the way, as a result of higher rotary speed of the electric motors at a request for further speeding up of railway vehicles nowadays, the rotary speed of revolutions of pinions is ever increasing. Therefore, when torque is transmitted between the gear wheel and the pinion, the above-mentioned tooth profile modifications bring about a decrease in the tooth bearing area, resulting in lowering of contact gear ratio. As a result, a problem arises in that an overall value of vibrations and noises that are generated at the time of gearing mesh of the gear wheel and the pinion becomes larger and also that such a main component of the overall value as is the frequency of the noises to be generated when the gear wheel and the pinion get into gearing mesh with each other is changing toward high-frequency ranges of 2000 to 3000 Hz. The noises at this kind of frequency band are most sensitive to the human acoustic senses on the loudness-level contours. As compared with the noises of the frequency band below 1000 Hz at the same phon unit, the human being feels the noises to be more noisy by about 10 times, thereby giving him/her discomfort.
Conventionally, as a method of reducing vibrations and noises that are generated when the gear wheel and the pinion come into gearing mesh with each other, there is known in Patent Document (1) the following art, namely, a method for performing a three-dimensional (3-D) bias tooth surface modification on the tooth surface of a helical gear, while leaving a complete contact line area of a width which is an integer multiple of a contact line pitch in the face width direction, such that contact does not occur on the tooth surface other than the complete contact line area at the time of gearing mesh (i.e., a 3-D tooth surface modification known as so-called bias-out is performed on the tooth surface in a manner in which the shape of tooth profile of the tooth surface in the tooth depth direction varies successively with the position in the flank line direction). Further, there is known in Patent Document (2) an art in which, an effective gearing mesh area of the tooth surface of a helical gear is subjected to crowning by an amount of 5 to 20 μm in the gear mesh contact line direction of the tooth surface. The tooth surface modifications are such that the maximum bias amount of modification becomes 10 to 40 μm after adding the above-mentioned crowning in the gear mesh contact line direction and the crowning for the purpose of addendum and dedendum modifications as well as flank line modification (i.e., a 3-D tooth surface modifications known as so-called bias-in is performed such that the shape of tooth profile of the tooth surface in the tooth depth direction successively varies with the position in the flank line direction).
However, since the methods in each of the above-mentioned patent documents performs the 3-D tooth surface modifications, the tooth surfaces can no longer be machined with widely used conventional gear grinding machines that are capable of performing 2-D tooth surface modifications. Therefore, expensive and high-performance gear grinding machines become necessary, thereby necessitating a vast amount of equipment investment. Further, machining for performing 3-D tooth surface modifications take longer time (at least more than 5 times of machining time) than the time for performing 2-D tooth surface modifications, thereby largely lowering the productivity. As a result, there is a problem in that the manufacturing of helical gears takes a vast amount of cost.